I need a formula that will display + or - in front of the value of my
current formula.

B4=7.5 B3=6.5

I want the result to display +15% or - if that were applicable.

One way is to use two steps.

First, use the formula:
=IF(B3=0,"-",B4/B3-100%)
(Notice the added check for dividing by zero.)

Second, select the cell holding the formula and use:
Format Cells Number Custom
and in the Type field put
+0%;-0%